1. FIELD OF THE INVENTION
This invention relates to an apparatus for lighting the discharge lamp of filament-preheating type, or more in particular to a switching apparatus comprising a semiconductor switching element for use with the lighting apparatus.
2. DESCRIPTION OF THE PRIOR ART
A conventional lighting apparatus for the discharge lamp of filament preheating type mainly employs a glow starter. The well-known lighting apparatus for the preheating-type discharge lamp using such a glow starter is shown in FIG. 1.
In FIG. 1, reference numeral 1 shows an AC power supply, numeral 2 a current limiter including a choke coil, numeral 4 a switching circuit, numeral 5 a discharge lamp of preheating type (hereinafter referred to only as the lamp), and numeral 3 a capacitor which may be replaced by an equivalent distributed capacity, if any, of the choke coil of the current limiter 2.
The glow starter is inserted in the switching circuit 4 which in turn is connected to those sides of the terminals a and b of the filaments F.sub.1 and F.sub.2 respectively which are opposite to the power supply 1. By causing a preheating current to flow in the filaments F.sub.1 and F.sub.2 of the lamp 5, a pulse voltage required to start the glow is generated thereby to detect the turning on or off of the lamp 5.
This conventional lighting apparatus using a glow starter, however, has the disadvantages that it requires a long time before the lamp is turned on and that the useful life of the glow starter is short. Especially, at and near the end of the life of the glow starter, the time required for the lamp to be turned on is as long as several to several tens of seconds. Also, the glow starter itself must be replaced by new one several times throughout the life of the lamp equipment.
A starter circuit for the discharge lamp of preheating type without the above-mentioned disadvantages is under development by utilizing the silicon symmetrical switch (SSS) or a silicon controlled rectifier (SCR) but has not yet been developed to a commercially effective point.